1. Field of the Invention
The present invention relates to inductive heaters, and in particular, to heaters working inside the bore of a workpiece.
2. Description of Related Art
Known inductive heaters typically have a large open coil for encircling a workpiece. These coils are designed to conduct very high currents and to magnetically stimulate the workpiece. The magnetically stimulated workpiece will be heated by hysteresis losses and the flow of large eddy currents.
To sustain the high currents needed for inductive heating, the primary inductive coil is typically rather large and requires an active cooling system. For example, known inductive coils have been hollow tubes through which a liquid coolant is circulated. The coolant can be provided from a refrigerant unit that keeps the coolant at a relatively low temperature.
Known inductive heaters have employed a parallel pair of inductors that conduct current in opposite directions. For example, in U.S. Pat. No. 4,788,396 a hairpin-shaped inductor can embrace a thin metal sheet covered with a metal powder that is to be sintered. This device works with relatively modest currents since the workpiece is less than 1 mm thick. Thus this reference does not have a coolant system. Furthermore, this reference is only concerned with an external inductor.
As a further example, in U.S. Pat. Nos. 3,699,302 and 3,836,743, bolts pass between a pair of parallel inductors to be inductively heated thereby. The latter two references are again concerned with external inductive heaters.
When cavities or bores are inductively heated, conventional apparatus have used a multi-turn coil. See for example U.S. Pat. No. 4,849,594. Because of the high currents and the multiple turns, these structures tend to be relatively bulky and are useful only for heating from inside a relatively large cavity. See also U.S. Pat. No. 4,625,090 revealing a relatively large and complicated current loop wound coaxially in the cylinder of a tripot.
U.S. Pat. No. 5,182,427 shows a parallel pair of inductors that are mounted inside a tube for conducting a loop of current. The inductors are mounted inside a number of spaced ferrite beads. The ferrite beads are designed to be heated by the magnetic fields of the parallel inductors. While this design does rely on the magnetic coupling to the ferrite beads, the heating of the surrounding body is not through inductive heating. Instead, the heating is the typical conductive, convective or radiant heating associated with prior art heaters. Also, this reference is unconcerned with high power applications where a coolant system is needed.
Inductive heating has been used in connection with the installation or removal of various fasteners. See U.S. Pat. Nos. 3,771,209 and 5,025,128.
See also U.S. Pat. Nos. 4,056,750; 4,695,712; 4,973,811; 5,066,755; 5,291,063; and 5,374,809.
In certain applications fasteners must be heated as part of the installation or removal procedure. For example, bolts used in industrial steam turbines are rather large steel devices that must be heated to lengthen the bolt. After cooling the bolt will contract and produce a high axial force for holding the turbine casing together. To facilitate this heating, these turbine bolts are made with an axial bore to hold a temporary heater element. Conventionally, a resistive heater wire has been inserted in the bore of the bolt. A disadvantage with this type of heater is the relatively long thermal delays needed to convey heat from the heater wire to the turbine bolt by conductive, convective and radiant processes. Because conventional inductive heaters are in the form of a helical coil, they have been unable to fit inside the relative slender bore of a turbine bolt.
Accordingly, there is a need for an inductive heater capable of efficiently heating a body having a slender bore.